struct student *creat() //1,创建链表,并且开始输入学生信息 
void zengjia(struct student *head) //2-增加学生信息 
void shanchu(struct student *head) //3-删除学生信息
void xiugai(struct student *head)//4-修改学生基本信息
void banjichengji(struct student *head)//5-班级内单科成绩排名
void yuwen(struct student *head)//5-1-语文单科成绩 
void shuxue(struct student *head)//5-2-数学单科成绩 
void yingyu(struct student *head)//5-3-英语单科成绩 
void lishi(struct student *head)//5-4-历史单科成绩 
void chengxu(struct student *head)//5-5-程序单科成绩 
void tiyu(struct student *head)//5-6-体育单科成绩 
void xueshengchengji(struct student *head)//6-查询学生信息 
void bujige(struct student *head)//7-统计所有不及格科目超过两科的学生名单
